1. Field of the Invention
This invention generally relates to systems configured for lens array-based illumination for wafer inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail (killer defect size also shrinks with the device size).
Uniform illumination fields for wafer inspection are typically generated via one of the following methods. For example, a uniform illumination field may be generated by one or more solid rectangular rods via total internal reflection from the sides of the rods for broad band electromagnetic spectra. In another example, a uniform illumination field may be generated by a single hollow light pipe with a relatively high reflective coating for narrow band electromagnetic spectra, typically at wavelengths less than 260 nm, where material absorption and optical damage become significant for optical materials. In an additional example, a uniform illumination field may be generated by diffractive optics with relay optics for narrow band electromagnetic spectra.
Illumination pupils (not necessarily top hat uniform pupil) for wafer inspection are typically generated via the following methods. For example, an illumination pupil may be generated by diffractive optics with relay optics for narrow band electromagnetic spectra. In another example, an illumination pupil may be generated with refractive condenser optics for wider band electromagnetic spectra. In an additional example, an illumination pupil may be generated with reflective optics, especially ellipsoidal optics for broadband electromagnetic spectra.
For wafer inspection, the following combinations are used to generate combined field and pupil. For narrow band, diffractive optics and refractive optics, for example, an aspherical lens with relay optics may be used to generate a pupil with a desired shape and non-uniformity followed by diffractive optics with relay optics to generate one or more rectangular uniform fields for narrow band electromagnetic spectra. For broad band, the combined field and pupil may be generated by a combination of refractive condenser optics and light pipe (solid or hollow) to form a uniform pupil and field. In another example for broad band, the combined field and pupil may be generated by an ellipsoidal reflector followed by a light pipe to form a pupil and uniform field. In an additional example for broad band, the combined field and pupil may be generated by an ellipsoidal reflector followed by light pipe, relay optics, and light pipe to form uniform pupil and field. In a further example for broad band, the combined field and pupil may be generated by an ellipsoidal reflector followed by a light pipe, micro lens array, and light pipe to form a uniform pupil and field.
There are, however, a number of disadvantages to the currently used optical configurations described above. For example, diffractive optics are in general for narrow band applications and are not suitable for broad band applications due to material dispersion. In addition, the refractive condenser plus light pipe combination has a limited light collection solid angle, e.g., ˜π due to the condenser numerical aperture (NA, e.g., NA<=0.9), as plasma sources are in general emitting in 4π solid angle. The working distance of the condenser may be too short to be used with a relatively high power laser induced plasma source. In another example, the collection solid angle of an ellipsoidal reflector can approach 3π. (The collection solid angle can, in theory, approach nearly 4π with two ellipsoidal mirrors, where a spherical mirror is considered a special ellipsoidal mirror. However, such a configuration is difficult to implement and the potential light gain may be limited due to plasma re-absorption.) However, the angular magnification of the ellipsoidal reflector with reasonable working distance can be substantially high such that the pupil generated by the ellipsoidal reflector alone will have substantially large non-uniformity. This non-uniformity is undesirable for machine-to-machine matching. In a further example, the rod plus relay optics (or lens array) plus rod approach for relatively high power, broad band deep ultraviolet (DUV) applications is in general undesirable due to thermal management, material handling, and material absorption and life time concerns.
Accordingly, it would be advantageous to develop systems for providing illumination for wafer inspection that do not have one or more of the disadvantages described above.